This invention relates to an improved process for the treatment of industrial wastewater. More specifically, the invention relates to an improved process for treating wastewater from industrial wastewater treatment plants using the biologically-activated sludge process in a manner that eliminates the need for any mechanical removal of excess solids, or the use of other conventional solids treatment units.
The production of excess solids in conventional biologically-activated sludge wastewater treatment plants is accepted as a normal part of operations but it can be a substantial and expensive problem. For example, even a small industrial wastewater pretreatment plant that receives influent hydraulic loading at the rate of only 35,000 gallons per day from a surfactant manufacturing operation might need to dispose of 300-400 tons of excess solids per year. These relatively large amounts of excess solids must be disposed of properly and legally and most often this disposal is done in landfills.
The aerobic microbiological treatment of waste organic matter from sewage or industrial effluents often uses the conventional biologically-activated sludge process. As used herein the biologically-activated sludge process is sometimes referred to as xe2x80x9cBASxe2x80x9d. A major drawback of this conventional technology is that the metabolic process of the biomass, the function that degrades and destroys the organic wastes, also produces solids growth as a normal consequence of cellular processes. Each pound of xe2x80x9cfoodxe2x80x9d (in the form of dissolved or suspended organic solids) that is consumed (converted) by the BAS process in the biomass can produce up to half a pound, or more, of inactive solids. These solids are dead and/or dying organisms of the biomass that have completed their biological cycle and are no longer effective. The accumulation of the excess solids appears in treated, wasted sludge mechanically removed from the treatment system. The routine disposal of the wasted sludge by dumping on land or at sea has the potential for adverse environmental effects and landfilling the treated sludge is increasingly more expensive, or in some extreme cases banned.
The BAS process has been used for many years for the treatment of wastewater, particularly for high-strength wastes. The BAS process maintains control over the beneficial use of the suspended biological solids in the biomass under aeration. The metabolic activity rate is determined by the presence of both sufficient dissolved oxygen and food. Aerobic bacteria, the predominant component of the biomass, feed on the dissolved organic wastes. The aerobic bacteria grow, reproduce and eventually die. The metabolic processes use dissolved oxygen and organic wastes found in wastewater to provide the fuel for cellular metabolism.
A drawback of the BAS process is that the total solids under suspension in the aeration tanks continue to increase until all the usable food is consumed, at which point the system dies, or goes septic. The only widely accepted solution to the solids buildup problem in plants using conventional BAS processes is the continual or intermittent mechanical removal of the solids from the system in order to maintain stable, optimal conditions for cell growth in the aeration tanks.
These relationship behaviors between available food and the amount of living biomass can be predicted mathematically as food/biomass ratios, referred to as F/M ratios or alternatively, as mean cell residence time""s (MCRT). Using these predictive behavioral indicators, wastewater operators can better control the BAS process and stabilize solids within an optimum range.
In a BAS process what remains of the untreated, dissolved solids in the finished water from the aeration tanks can usually be settled out by gravity (sometimes with the help of polymers) in a secondary clarifier as the sludge blanket. The portion of the sludge blanket which is not returned to the aeration step in the BAS process as return activated sludge is transferred as waste-activated sludge to a solids treatment process. xe2x80x9cTreatmentxe2x80x9d or xe2x80x9cshotxe2x80x9d tanks are used for staging waste-activated sludge before it is sent to a mechanical water removal and/or drying unit. In these staging tanks, strong inorganic chemicals, ferric chloride and lime, dramatically lower the wastewater""s pH and aid in the shearing of the filaments of activated sludge. Usually a small amount of a flocculation polymer is added to the treated sludge to facilitate the formation of particles of sheared sludge of the proper size and shape until the point at which optimal settling out occurs.
The clear water above the thickened, polymer treated sludge layer is decanted from the tops of these tanks and directly discharged or recycled. The thickened sludge layer left in the bottom of the treatment tanks is pumped to a mechanical dewatering device, usually a filter press, where water is mechanically squeezed out leaving the modified, dewatered sludge in the form of a filter cake. This filter cake is accumulated on site and then transported to a landfill.
In conventional BAS processes, the aerobic digestion process almost never goes to completion; that is, all of the food (including the excess solids, or sludge) is consumed. When biological processes achieve 100% completion, the only byproducts are H2O, CO2, and traces of inert ash. However, to digest (biotreat) a single pound of food in a conventional BAS process as much as 0.53 lbs. of dead/dying bugs can be created. Thus, the total amount of solids under aeration slowly increases and ratio of living to dead organisms in the biomass slowly decrease. Soon total solids are so concentrated in the aeration tanks that the aerobic process is greatly inhibited. Therefore, excess solids have to be mechanically removed at that point in order for effective bioactivity to resume.
Various processes have been proposed to minimize or eliminate excess sludge in conventional BAS wastewater treatment processes. One such wastewater treating process is set forth in U.S. Pat. No. 5,356,537 to Thurmond et al. The Thurmond process teaches a process for treating wastewater including mixing wastewater with activated sludge in an aeration tank followed by clarifying in a settling tank. From 5% to 25% of the activated sludge solids from the settling tank is returned to the aerobic digester. The activated sludge solids is held in the aerobic digester from about 16 to 24 hours and then the activated sludge is transferred from the aerobic digester to the aeration tank. This process must employ both multiple cycles as well as multiple treatment units to be effective in the removal of excess solids (sludge) from the system.
In U.S. Pat. No. 3,047,492 to Gambrel there is described a modified activated sludge type of sewage treatment that eliminates the sludge beds, anaerobic digesters and costly sludge handling equipment. The Gambrel invention is said to eliminate the production of sludge and produce a clear final effluent that can be safely discharged into a creek or stream. To eliminating sludge, the Gambrel process incorporates a separate stage wherein most of the sludge from the settling tanks and approximately 25% of the daily intake of the plant is returned to the aeration tanks of the system while approximately 3% thereof is diverted to an aerobic digester. In the aerobic digester the sludge is detained for approximately 10 to 15 days and completely digested to produce an inert invisible ash and clear liquid that is then returned to the system. Once again, multiple units, as well as longer multiple cycles, would be required for the effective reduction in total sludge discharged.
Thus, there remains an unfilled need for an improved, simplified BAS process for treating wastewater to eliminate excess sludge quickly, efficiently, economically and without the use of both multiple treatment units and multiple, time-consuming, extra cycles. This improved process should also be useful in the treatment of lower-strength wastewater in other sewage treatment plants that use the BAS process.
It is therefore a general object of the present invention to provide improvements to the conventional BAS process that eliminate excess solids from industrial wastewater without using any mechanical means for their removal.
In the process of this invention batch-recycling loops are added to the conventional serial or continuous flow BAS process. Using novel mechanisms, excess solids can be almost completely eliminated from the waste generated by the normal biological processes during the operation of this improved BAS process.
Industrial wastewater from a manufacturing plant is first screened to remove insoluble, mostly inorganic waste, trash and other larger pieces of untreatable debris. The wastewater flow is then measured. Before it is introduced into the wastewater treatment plant, the wastewater enters a properly sized, specialized tank which slows the water flow. This tank performs some of the functions of a clarifier. For example, it provides for the removal of insolubles, such as fats, oils, and greases.
In the process of this invention, the process wastewater first undergoes an equalization process that smoothes hydraulic flow. In the equalization tanks, the pH of the wastewater is continuously monitored and automatically adjusted to maintain near neutral conditions; that is, pH in the 6.8-7.2 range that avoids shocking the biomass in the next step. The equalized wastewater is then transferred to aeration tanks where the dissolved organic wastes (mostly high molecular weight solids) are absorbed, or metabolized, or converted by active biodegradation into smaller, simpler, easier to metabolize organic materials. Eventually, almost all of these are converted to CO2, H2O and inert ash by the microorganisms in the biomass.
After the aeration step, the finished wastewater is transferred to a clarifier which uses gravity to separate the wastewater into two distinct phases; a liquid phase, and a semi-solid phase, or sludge, which settles to the bottom of the clarifier and forms a blanket. Small doses of polymers may be added to the clarifier to facilitate the agglomeration of sludge into this semi-solid waste state. The liquid phase may be discharged from the clarifier for further treatment, or, in some cases, directly discharged as effluent. Some of the semi-solid phase, or sludge blanket, is transferred to a bioreactor or aerobic digester, as waste-activated sludge where it is closely monitored, and kept alive in the virtual absence of food by a controlled recycling and aggressive aeration process.
The remainder of the sludge blanket is recycled to the aeration tanks as return-activated sludge where it is mixed with newly introduced untreated wastewater. The combination, called xe2x80x9cmixed liquor,xe2x80x9d undergoes a new cycle of aerobic, microbial metabolic treatment. As a result of the controlled recycling of hungry aggressive digester sludge into the biomass that has been recharged with fresh food-containing wastes, the excess solids normally generated in conventional BAS processes is maintained at or near zero.
In the process of this invention, the waste-activated sludge is closely monitored, and kept under more active aeration than in conventional BAS processes. The modified wastewater treatment system of this invention is a semi-continuous flow systemxe2x80x94as opposed to the completely continuous flow of most conventional treatment systems. The manner in which batch recycle-loops are used in this new process controls the biomass differently, and more effectively, than conventional BAS processes. This difference results in a substantial reduction of the uncontrolled buildup of solids under aeration, virtually eliminating the need for mechanical removal of solids from the system to maintain equilibrium. The method of this invention results in the consumption of almost all of the available food and assures that the biomass consumes almost all of its own dead/dying members as part of that foodxe2x80x94the ultimate cause of the excess solids growth problem in the first place.